It's a great free expansion to an already great game. The new island has some cool Bugsnax and additional character development, the housing system and the associated mail quests gives you a lot more stuff to do and get to know Grumpuses even better, and the additional lore continues the trend of being surprisingly fucked up for a kid's game.
Only issue I have is that the "feed X different types of Y bugsnax to grumpuses" quests suck, especially when you have no clue if you're just one transformation off or if it turns out one of the bugsnax doesn't actually count towards it.